There are many limitations in ground water cleanup programs, which limiting factors are considered in the lifetime costs of implementing remedial actions (O'Brien et al, 1997). For this reason, permeable reactive barriers and funnel and gate systems are currently the most cost effective methods for cleaning ground water.
There are currently several chemical oxidation systems in which soil for aquifer material has been remediated using hydrogen peroxide in the Fenton mechanism. These systems include injecting hydrogen peroxide in situ, surface soil application and soil slurry reactors. Each of these systems has potential limitations which ultimately affect the feasibility of the system in treating groundwater.
Blowes et al., in U.S. Pat. Nos. 5,362,394 and 5,514,279, disclose treating contaminated water by excavating a trench in the aquifer in the path of a contaminant plume, and placing a body of active material which causes the contaminant, by chemical reaction, to change its oxidation-reduction state and to precipitate harmlessly in the body of the material. This process merely involves flowing waste through the active material without concentrating the contaminant.
Doddema et al., in U.S. Pat. No. 5,667,690, disclose a process tor treating contaminated water involving a complex of a transition metal and a polyamine in the presence of a peroxide. Doddema et al. propose that in situ treatment of contaminated water involves passing a solution of an iron-polyamine complex through the soil or material in such a way that all soil particles come sufficiently in contact with the iron-polyamine complex and peroxide. This approach essentially involves injecting a mixture of the peroxide and iron-polyamine complex into the subsurface. This involves oxidation of contaminants in the subsurface, as is. There is no adsorption of the contaminants from the aqueous phase onto reactive media followed by oxidation of the contaminants on or near the surface on which the contaminant was concentrated. Additionally, the iron-polyamine complex is not fixed to the surface of carbon particles. According to Doddema, transition metals such as iron, manganese, and cobalt can be used, and the reactions are conducted at a pH of 9.5.
Gurol, in U.S. Pat. No. 5,755,977, discloses oxidation of target contaminants in the aqueous phase. In this patent, iron is used in the mineral form as goethite, and the pH range is 5-9.
Jasim et al., in U.S. Pat. No. 5,716,528, discloses a homogeneous (aqueous phase) reaction of Fe+2 and hydrogen peroxide to oxidize pentachlorophenol. Activated carbon is used merely for post-oxidation treatment, i.e., as a polishing step. Adsorption and oxidation occur in sequential treatment reactors.
Gilham, in U.S. Pat. No. 5,534,154, teaches primarily reductive (dehalogenation) reactions which are quite different from oxidation reactions. Reduction involves donating an electron from a reduced moiety, e.g., Fe0, to a compound (contaminant).
Leachate generation is a potential limitation in surface soil application of hydrogen peroxide, resulting in the downward transport of contaminants. In soil slurry reactors, the treatment volume of contaminated soil is generally small, representing a limitation to the overall treatment process.
Competition kinetics can significantly reduce treatment efficiency and minimize effectiveness when scavengers react with hydroxyl radicals more rapidly than the target compound, as shown in Table 1. Scavenging can be minimized by using low ionic strength or low hardness make-up water for the hydrogen peroxide solution.
TABLE 1Chemical Reactions Involving H2O2, Iron, 4-POBN,2CP and ScavengersH2O2 + Fe(II) → Fe(III) + OH− + •OH (1)H2O2 + Fe(III) → Fe(II) + •O2− + 2H+(2)4-POBN + •OH → •4-POBN(3)2CP + •OH → reaction products(4)Σni − 1ki•OH + Σni − 1Si  → products of scavenging reactions(5)•O2− + Fe(III) → Fe(II) + O2(6)H2O2 + 2Mn(II) + 2H2O → 2MnOOH(s) + 4H+(7)H2O2 + 2MnOOH(s) + 4H+ → 2Mn(II) + O2 + 4H2O(8)catalaseH2O2 + O2 + 4H2O(9)where•OHhydroxyl radical•O2−superoxide radical4-POBNspin-trap compound•4-POBNradical adduct2CP2-chlorophenolSiconcentration of individual scavengerskisecond-order rate constant (L/mol-s) for•OH with SiReactionReaction Rate Constant and General Comments1ki = 53•01 l?mol-s (Ingles, 1972), 76 L/mol-s(Walling, 1975)2Rate constant not reported; reaction involvessoluble and solid phase iron3k3 = 3.8 × 109 L/mol-s, pH 7 (Buxton et al.,1988)4k4 = 1.2 × 1010 L/mol-s (Getoff and Solar, 1986)5Σni − 1ki[Si] - pseudo-first-order rate constant(T−1) for •OH scavenging by all constituents ofthe solution except the probe6k6= 2.7 × 108 L/mol-s
Limited reaction kinetics is the condition in which low concentrations of the target compound limits the second-order oxidation reaction. Correspondingly, the clean-up goal for the target compound in the ground water can be difficult to achieve. Exacerbating the issue are the numerous scavengers which effectively compete against low concentrations of target compound for hydroxyl radicals.
While adsorption using activated carbon and oxidation using the Fenton mechanism has been widely used separately in ground water remediation and wastewater treatment, problems associated with oxidation in subsurface systems involve poor reactions kinetics, excessive scavenging and excessive non-productive hydrogen peroxide consuming reactions. Problems associated with adsorption in subsurface systems relate to exhausting the sorption capacity of carbon. To replace the carbon, it must be excavated and transported To a specialized facility for disposal. Long-term risks associated with this disposal are environmentally undesirable. If the carbon is reactivated rather than disposed of, additional costs are incurred.
Enzymatic and manganese reactions with hydrogen peroxide can consume hydrogen peroxide in reactions which do not yield hydroxyl radicals (cf. Table 1 and FIG. 2). Selection criteria for granulated activated carbon should, therefore, include low manganese content. The iron content of the granulated activated carbon can be increased to enhance the Fenton mechanism. The effect of the enzymatic reactions are relatively short term because hydrogen peroxide inhibits catalase enzyme activity via the formation of an intermediate-enzyme-substrate compound (Nicholls and Schonbaum, 1963; Aggarwal et al., 1991).